The present invention relates to a method and an apparatus for failsafe monitoring of a movement variable on an electrical drive. In particular, the invention relates to failsafe monitoring of an electrical drive for safeguarding a dangerous area on an automatically operating machine or installation, such as a machine tool, a robot, a conveyor belt or a door which opens and closes automatically.
Such danger areas of machines or installations are often safeguarded by protective fences, light barriers and other devices which prevent direct access to the danger area and/or shut down the machine or installation when the danger area is entered. However, situations exist in which access to a danger area cannot be entirely prevented, for example when setting up an automated process sequence on a machine, or in the case of an automatic door. In order to minimize the risk of injury to people even in such situations, it is known to limit movement variables of the drives causing the danger, such as a rotational speed and/or the torque of the drive, to a defined maximum value. For example, a machine tool can be operated at a reduced, limited speed when a protective door is open. In order to ensure the safety of people in these situations, the dangerous movement variables of the operating drive must be monitored in a failsafe manner.
Failsafe monitoring is usually implemented by monitoring the movement variable at least in a duplicated form, and by comparing the respective monitoring results with one another. The dangerous movement of the drive is permitted only when and for as long as the redundant monitoring results correspond.
DE 100 35 738 A1 proposes a redundant safety device for monitoring the rotation speed of a synchronous drive or of an asynchronous drive, with the rotation on the one hand being detected by a rotation-speed sensor which is connected to a drive shaft. The rotation-speed sensor generates the instantaneous rotation speed as an input variable for a drive control loop. In addition, the drive currents are measured in two of the three so-called phases of the drive. The measured drive currents are converted into a field-producing direct-axis current component and a torque-producing quadrature-axis current component. A plausibility comparison of the current values with acceleration values, which are determined using the rotation-speed signals from the rotation-speed sensor, makes it possible to detect faults in the redundant monitoring system.
DE 101 63 010 A1 discloses a further apparatus and a further method for failsafe monitoring of the speed of an electrical drive. The apparatus has two processors, which allow a cross-comparison of the monitoring results, with the two processors carrying out the monitoring in different ways. The first processor processes the regular control algorithm for closed-loop rotation-speed control, and it uses an estimated or measured rotation-speed value to carry out the monitoring. The second processor uses measured current values or a reconstruction of the control voltage in order to determine an instantaneous output frequency of the so-called frequency converter, which produces the drive currents. This method and this apparatus are intended to make it possible to dispense with a separate rotation-speed sensor. A similar apparatus is also described in DE 10 2005 045 284 A1.
The above apparatuses and methods are primarily used for monitoring a rotational speed, and thus for monitoring a speed of the drive. Monitoring for maintaining a defined torque is not provided.
DE 42 34 501 A1 discloses a method and an apparatus for redundant monitoring of the maximum torque of an electrical drive. In one channel, the instantaneous torque is determined using the drive currents. In a second channel, the instantaneous torque is estimated using an instantaneous rotation speed and using characteristics of the electrical drive, with the rotation speed again being determined by means of a rotation-speed sensor. This known method accordingly requires a separate rotation-speed sensor for torque monitoring.